This invention relates generally to a method and apparatus for detecting a received signal in a communication system. More particularly, this invention relates to a method and apparatus for detecting locations of path rays in a multi-path receiver having multiple time references.
FIG. 1 is a block diagram of an exemplary cellular radiotelephone system, including an exemplary base station (BS) 110 and a mobile station (MS) 120. Although denoted a “mobile station”, the station 120 may also be another type of remote station, e.g., a fixed cellular station. The BS includes a control and processing unit 130 which is connected to a mobile switching center (MSC) 140 which in turn is connected to a PSTN (not shown). General aspects of such cellular radiotelephone systems are known in the art. The BS 110 handles a plurality of voice channels through a voice channel transceiver 150, which is controlled by the control and processing unit 130. Also, each BS includes a control channel transceiver 160, which may be capable of handling more than one control channel. The control channel transceiver 160 is controlled by the control and processing unit 130. The control channel transceiver 160 broadcasts control information over the control channel of the BS or cell to mobiles locked to that control channel. It will be understood that the transceivers 150 and 160 can be implemented as a single device, like the voice and control transceiver 170, for use with control and traffic channels that share the same radio carrier.
The MS 120 receives the information broadcast on a control channel at its voice and control channel transceiver 170. Then, the processing unit 180 evaluates the received control channel information, which includes the characteristics of cells that are candidates for the MS to lock on to, and determines on which cell the MS should lock. Advantageously, the received control channel information not only includes absolute information concerning the cell with which it is associated, but also contains relative information concerning other cells proximate to the cell with which the control channel is associated, as described for example in U.S. Pat. No. 5,353,332 to Raith et al., entitled “Method and Apparatus for Communication Control in a Radiotelephone System”.
Modern communication systems, such as a cellular radiotelephone system of the type described above and satellite radio systems, employ various modes of operation (analog, digital, dual mode, etc.) and access techniques such as frequency division multiple access (FDMA), time division multiple access (TDMA), code division multiple access (CDMA), and hybrids of these techniques.
In North America, a digital cellular radiotelephone system using TDMA is called the Digital Advanced Mobile Phone System (D-AMPS), some of the characteristics of which are specified in the TIA/EIA/1S-136 standard published by the Telecommunications Industry Association and Electronic Industries Association (TIA/EIA). Another digital communication system using direct sequence CDMA is specified by the TIA/EIA/1S-95 standard. There are also frequency hopping TDMA and CDMA communication systems, one of which is specified by the EIA SP 3389 standard (PCS 1900). The PCS 1900 standard is an implementation of the GSM system, which is common outside North America, that has been introduced for personal communication services (PCS) systems.
Several proposals for the next generation of digital cellular communication systems are currently under discussion in various standards setting organizations, which include the International Telecommunications Union (ITU), the European Telecommunications Standards Institute (ETSI), and Japan's Association of Radio Industries and Businesses (ARIB).
Direct-sequence (DS) spread-spectrum modulation is commonly used in CDMA systems, in which each information symbol is represented by a number of “chips.” Representing one symbol by many chips gives rise to “spreading,” as the latter typically requires more bandwidth to transmit. The sequence of chips is referred to as the spreading code or signature sequence. At a DS receiver, e.g., a Rake Receiver, the received signal is despread using a despreading code, which is typically the conjugate of the spreading code. IS-95 and J-STD-008 are examples of DS CDMA standards.
In the mobile radio channel, multi-path is created by reflection of the transmitted signal from obstacles in the environment, e.g., buildings, trees, cars, etc. In general, the mobile radio channel is a time varying multi-path channel due to the relative motion of the structures that create the multi-path.
A characteristic of the multi-path channel is that each path through the channel may have a different phase. For example, if an ideal impulse is transmitted over a multi-path channel, each pulse of the received stream of pulses generally has a different phase from the other received pulses. This can result in signal fading.
In a CDMA system, signal fading is combated by combining the pulses received over multiple paths using a Rake Receiver. Typically, the channel is modeled as discrete MS. The locations of the received signal path rays are first found by using a searcher, and then these path rays are combined by using a maximum ratio combiner. A tracker is used to track the locations of the path rays.
FIGS. 2A and 2B illustrate a typical DS-CDMA system including a Transmitter 200 and a receiver 250, respectively. The Transmitter 200 includes a Spreader 210, a Modulator 220, and an antenna. The Spreader 210 spreads the signal to be transmitted, and the Modulator 220 modulates the spread signal on a carrier frequency. The modulated signal is then transmitted via the antenna in the Transmitter 200.
The Receiver 250 includes an antenna, an RF Pre-Processing unit 230, a Rake Receiver 240, and a Post-Processing unit 250. The transmitted signal is received via the antenna in the Receiver 250. The RF Pre-Processing unit 230 tunes to the desired band and desired carrier frequency, then demodulates, amplifies, mixes, and filters the signal down to baseband.
The Rake Receiver 240 despreads the demodulated signal and detects the digital symbols that were transmitted. It may produce soft information as well, which gives information regarding the likelihood of the detected symbol values.
The Post-Processing unit 250 performs functions that depend highly on the particular communications application. For example, it may use the soft detected values to perform forward error correction decoding or error detection decoding. It may convert digital signals into speech using a speech decoder.
An MS, in the active mode, transmits and receives data. In the active mode, the MS uses a crystal oscillator with high accuracy and low phase noise as a reference clock. For instance, in the Global System for Mobile Communications (GSM), it is proposed to use a 13 MHZ crystal with high accuracy during the time the MS is awake. This crystal is synchronized with the BS to provide synchronous transmission and reception. This kind of crystal consumes a relatively large amount of current and is not necessary when the MS is, e.g., asleep in the idle mode, not receiving or transmitting data.
The MS only wakes up once in a while in the idle mode to listen to the Paging Channel (PCH) in order to receive paging messages from the BS. Hence, a crystal with low accuracy and low current consumption can be employed when the MS is asleep. An example of such a crystal is a Real Time Crystal (RTC), which is a 32 KHz crystal. An RTC has high phase noise and is much more sensitive to temperature variations than, e.g., a 13 MHz crystal. Employing this crystal is feasible in the sleep mode since the MS needs only to have knowledge about the time.
When the MS wakes up to listen to the PCH, the crystal with high accuracy and also high current consumption can be used. Since the wake up time is only a small part of the total time (if the MS is not paged), by switching between these crystals the MS can save power, thereby increasing its standby time.
It is known to use different clocks in different modes. For example, GB 232 4681 discloses a method for conserving energy by entering a lower power sleep mode. A clock with coarse resolution is used while the MS is in an idle mode. An offset between this clock and a fine resolution clock is determined. This offset is adjusted when the MS exits the low power sleep mode. The sleep time is measured using the coarse resolution clock so that the MS prewakes up to initiate matching signals to the pseudorandom noise (PN) code.
While switching from the crystal with low accuracy to the crystal with high accuracy conserves current, this affects the locations of the path rays. Due to the inaccuracy of the crystal which is used during the idle mode, the locations of the path rays may be lost when the MS is asleep during the idle mode and thus may not be able to be tracked when the MS wakes up again. In a CDMA system, this is considered to be a dilemma since the locations of the path rays are not trackable any longer, and the searcher must be activated every time the MS wakes up to locate the path rays.
Techniques have been proposed for detecting locations of path rays. For example, U.S. Pat. No. 5,790,589 discloses a method for locating a path ray by locating the PN code phase offset by initiating a search window on its expected location. The search window is re-located if the code phase offset is not detected. This patent does not address the use of multiple time references.
There is thus a need for an efficient and fast method for detecting the location of path rays in a multi-path receiver using multiple time references.